JP7182010B2 - Threaded fittings for steel pipes - Google Patents

Description

The present disclosure relates to threaded joints used for connecting steel pipes.

In oil wells, natural gas wells, etc. (hereinafter also collectively referred to as "oil wells"), in order to extract underground resources, a casing that constructs a multi-stage well wall, and a casing that is arranged in the casing and contains oil and gas Tubing is used that produces These casings and tubings are formed by connecting many steel pipes in sequence, and threaded joints are used for the connections. Steel pipes used in oil wells are also called oil well pipes.

Types of threaded joints are roughly classified into an integral type and a coupling type.

In the integral type, oil country tubular goods are directly connected to each other. Specifically, one end of the oil country tubular good is provided with a female threaded portion, and the other end thereof is provided with a male threaded portion. concatenated.

In the coupling type, oil country tubular goods are connected to each other via tubular couplings. Specifically, both ends of the coupling are provided with female threads, and both ends of the oil well pipe are provided with male threads. One male threaded portion of one oil country tubular good is screwed into one female threaded portion of the coupling, and one male threaded portion of another oil country tubular good is screwed into the other female threaded portion of the coupling. are concatenated. That is, in the coupling type, of a pair of pipe members to be connected, one pipe member is an oil well pipe and the other pipe member is a coupling.

In general, the end of an oil country tubular good having a male thread is called a pin because it includes an element that is inserted into a female thread formed in the oil country tubular good or coupling. An end of an oil country tubular good or a coupling formed with an internal thread is called a box because it includes an element that receives an external thread formed on the oil country tubular good.

In recent years, further development of high-temperature, high-pressure deep wells is progressing. Deep wells usually use thick-walled oil country tubular goods having high pressure resistance. In addition, due to the complexity of the depth distribution of formation pressure, it is necessary to increase the number of stages in the casing. can be These thick joints and slim joints are not only required to have high strength and sealing performance, but are also subject to severe restrictions on the outer diameter due to the multiple arrangement of oil country tubular goods.

In thick joints and slim joints with large dimensional restrictions, Patent No. 5908905 (Patent Document 1), International Publication No. 2017/104282 (Patent Document 2) and International Publication No. 2015/194160 (Patent Document 3) ), there are many designs in which an intermediate shoulder surface and an intermediate seal are provided in the axial middle of the joint, and the male and female threads are formed by a two-step screw with threads arranged in front and behind the shoulder face and the intermediate seal. . In a threaded joint having a structure in which the tip of the pin also has an inner seal portion, the intermediate seal portion is provided mainly to ensure sealing performance against external pressure.

In threaded joints of this design, there are critical cross sections (PICCS and BICCS) in the middle of the joint, as also disclosed in the above-mentioned US Pat.

The critical cross-section (CCS) is the cross-section of the joint portion that has the smallest area to withstand tensile loads in the closed state. When an excessive tensile load is applied, there is a high possibility that fracture will occur at the location of the critical cross section.

When the male and female threads are composed of a general single-step thread, the box at the root position of the female thread at the tip end of the male and female threads in the engaged state. is the dangerous cross section. The ratio of the critical cross-sectional area CCSA to the cross-sectional area A of the main body of the oil country tubular good (CCSA/A) is called the joint efficiency, and is widely used as an index of the tensile strength of the joint relative to the tensile strength of the main body of the oil country tubular good. Coupling-type threaded joints are sometimes designed so that the critical cross-sectional area CCSA of the coupling is larger than the cross-sectional area A of the main body of the oil country tubular good and less than 110% of the cross-sectional area A of the main body of the oil country tubular good. many.

On the other hand, in a threaded joint having a two-step thread structure, as described above, there is also a cross section in the axial center of the joint portion, which has a smaller area to withstand a tensile load, and this cross section is the intermediate critical cross section (ICCS). It is said.

Japanese Patent No. 5908905 WO2017/104282 WO2015/194160

In threaded joints with a two-step thread structure that has an intermediate shoulder surface and an intermediate seal, the area of the pin intermediate critical cross section (PICCS) (PICCSA: Pin Intermediate Critical Cross Section
Area) and Box Intermediate Critical Cross Section Area (BICCSA) change, the proportion of the load distributed to each critical cross section when various combined loads act changes, and the critical cross section The behavior of extension deformation and bending deformation in the vicinity changes. As a result, the sealing performance of the intermediate seal portion located between the pin and box intermediate critical cross-sections is affected.

When PICCSA+BICCSA is constant, if PICCSA is too large, BICCSA becomes too small. For example, when a tensile load is applied, the amount of elongation deformation of the box near BICCS increases. As a result, the seal surface of the intermediate seal portion of the pin and the seal surface of the intermediate seal portion of the box are misaligned in the axial direction. Decreased performance. In addition, a lower BICCSA means that the coupling is thinner near the end of the coupling than at the mid-shoulder, i.e., less rigid near the mid-seal, resulting in a lower BICCSA under combined loading. Bending deformation and the like are likely to occur.

On the other hand, if the PICCSA is too small, the extension deformation amount of the pin near the PICCS increases. As a result, the seal surface of the intermediate seal portion of the pin and the seal surface of the intermediate seal portion of the box are misaligned in the axial direction. Decreased performance.

In this way, the sealing performance varies depending not only on the sum of the areas of PICCS and BICCS but also on how the areas are distributed between PICCS and BICCS.

In Patent Document 1, while maximizing the shoulder area to obtain more torque resistance, the area of the CCS is within ± 5% of the sum of the areas of the PICCS and the BICCS. Threaded joints are disclosed that can balance the efficiencies between them, thereby maximizing the efficiency of the connection, as well as ensuring axial performance of the connection.

Patent Document 2 above discloses that the amount of interference between the intermediate shoulder surfaces of the pin and the box is appropriately set to ensure sealing performance and suppress the occurrence of crevice corrosion.

In Patent Document 3, by providing a nose portion connected to the inner seal surface between the inner shoulder surface of the tip of the pin and the inner seal surface, the elastic recovery of the nose portion increases the effect of amplifying the contact surface pressure between the inner seal surfaces. It is disclosed to obtain a high sealing performance mainly against internal pressure.

However, none of the above Patent Documents 1 to 3 disclose the effect of the area ratio of PICCS and BICCS on the sealing performance of a threaded joint.

An object of the present disclosure is to ensure excellent sealing performance in a threaded joint for steel pipes having a double thread structure including an intermediate seal and an intermediate shoulder.

A threaded joint for steel pipes according to the present disclosure includes a tubular pin and a tubular box, and the pin is screwed into the box to fasten the pin and the box. The pin includes a first male threaded portion, an intermediate seal portion, and a second male threaded portion in order from the tip end side of the pin, and a tip end portion is provided between the first male thread portion and the intermediate seal portion of the pin. A facing intermediate shoulder surface may be provided. The box includes, in order from the innermost side of the box, a first female threaded portion into which the first male threaded portion of the pin is fitted in a fastened state, an intermediate seal portion into which an intermediate seal portion of the pin is fitted in a fastened state, and A second female threaded portion may be provided in which the second male threaded portion of the pin is fitted in a fastened state, and an intermediate shoulder surface may be provided between the first male threaded portion and the intermediate seal portion of the box. An intermediate shoulder surface of the box axially opposes an intermediate shoulder surface of the pin. The pin may further include a pin intermediate risk section located near the end on the proximal side (that is, the end on the side opposite to the distal side) of the first male threaded portion. The box may further include a box dangerous cross-section located near the far end of the first female thread and a box intermediate dangerous cross-section located near the far end of the second female thread. .

PICCSA is the area of the pin intermediate risk section when the pin and the box are not fastened, BICCSA is the area of the box intermediate risk cross section when the pin and the box are not fastened, and the pin and the box When BCCSA is the area of the box critical cross-section when the is not fastened, the pin and the box have the following relationship:
PICCSA + BICCSA > BCCSA
0.70≦PICCSA/BICCSA≦0.95
is preferably satisfied.

Advantageous Effects of Invention According to the present disclosure, excellent sealing performance can be ensured in a threaded joint for steel pipes having a two-step thread structure including an intermediate seal and an intermediate shoulder.

FIG. 1 is a longitudinal sectional view showing a threaded joint for steel pipes according to an embodiment. FIG. 2 is a graph showing the relationship between the seal performance evaluation value and the area ratio of the intermediate critical cross-sectional area of the pin to the intermediate critical cross-sectional area of the box. FIG. 3 is a graph showing the relationship between the sum of the ratio of the intermediate dangerous cross-sectional area of the pin to the cross-sectional area of the main body of the steel pipe and the ratio of the intermediate dangerous cross-sectional area of the box, and the seal performance evaluation value.

A threaded joint for steel pipes according to the present embodiment includes a tubular pin and a tubular box. The pin and box are fastened together by screwing the pin into the box. The pin may be provided at the end of a steel pipe such as an oil well pipe. The box may be provided at the end of the coupling or at the end of another steel pipe.

The pin can have, in order from its distal end, a nose portion, an inner seal portion, an inner male thread portion, an intermediate shoulder portion, an intermediate seal portion, and an outer male thread portion. Note that the nose portion and the inner seal portion may not be provided. In addition, the tip surface of the pin constitutes an inner shoulder surface. When a nose portion is provided, the tip surface of the nose portion constitutes the inner shoulder surface. When the nose portion is not provided and the tip portion of the pin is formed by the inner seal portion, the tip surface of the inner seal portion forms the inner shoulder surface. If neither the nose portion nor the inner seal portion is provided, the inner shoulder surface can be formed by the tip surface of the inner male threaded portion or the tip surface of another portion extending from the inner male threaded portion toward the tip side. The inner shoulder surface of the pin faces axially distally.

An inner male threaded portion is provided between the inner seal portion and the intermediate shoulder portion. The inner male threaded portion may have an outer peripheral surface whose diameter gradually decreases toward the distal end side. The outer peripheral surface of the inner male threaded portion may be provided with a tapered thread whose diameter gradually decreases toward the distal end side.

An intermediate shoulder portion is provided between the inner male threaded portion and the intermediate seal portion. The intermediate shoulder portion may include an intermediate shoulder surface that is orthogonal or tapered to the axial direction of the pin. The intermediate shoulder surface may face axially distally.

The intermediate seal portion is provided between the intermediate shoulder portion and the external male thread portion.

The external male threaded portion may have an outer peripheral surface whose diameter gradually decreases toward the distal end and whose outer diameter at the distal end is larger than that at the proximal end of the internal male threaded portion. The outer peripheral surface of the external male thread portion may be provided with a tapered thread whose diameter is gradually reduced toward the distal end side.

The box is, in order from the back side, a concave portion corresponding to the nose portion of the pin, an inner seal portion corresponding to the inner seal portion of the pin, an inner female thread portion corresponding to the inner male thread portion of the pin, and an intermediate shoulder portion of the pin. There may be an intermediate shoulder portion, an intermediate seal portion corresponding to the intermediate seal portion of the pin, and an external internal thread portion corresponding to the external external thread portion of the pin. The concave portion can be configured such that the nose portion is inserted therein in the fastened state and a gap is formed between the nose portion and the concave portion. In this case, the inner shoulder surface can be formed by the inner side end surface of the recess. It should be noted that if the pin does not have a nose, it is preferable not to provide the recess. Also, if the pin does not have an inner seal, it is preferable that the box does not have an inner seal.

An inner female thread is provided between the inner seal portion and the intermediate shoulder portion. The inner female threaded portion has a structure corresponding to the inner male threaded portion of the pin, and may have an inner peripheral surface whose diameter gradually decreases toward the inner side of the box. The inner peripheral surface of the inner female threaded portion can be provided with a tapered thread whose diameter is gradually reduced toward the inner side of the box.

An intermediate shoulder portion is provided between the internal threaded portion and the intermediate seal portion. The intermediate shoulder portion may include an intermediate shoulder surface that is orthogonal to the axial direction of the box or conical. The intermediate shoulder surface axially opposes the intermediate shoulder surface of the pin.

The intermediate seal portion is provided between the intermediate shoulder portion and the external female thread portion.

The outer female thread has a structure corresponding to the outer male thread of the pin, and gradually decreases in diameter toward the back of the box. can have an inner peripheral surface with a larger diameter than the The inner peripheral surface of the outer female threaded portion may be provided with a tapered thread whose diameter is gradually reduced toward the inner side of the box.

In the fastened state, the nose of the pin is inserted into the recess of the box. In the fastened state, the inner seal portion of the pin fits into the inner seal portion of the box with an interference fit. In the fastened state, the inner male threaded portion of the pin is screwed into the inner male threaded portion of the box. In the fastened state, the intermediate shoulder surface of the intermediate shoulder portion of the pin axially opposes the intermediate shoulder surface of the intermediate shoulder portion of the box. Preferably, in the fastened state the intermediate shoulder surface of the intermediate shoulder of the pin contacts the intermediate shoulder surface of the intermediate shoulder of the box. In the fastened state, the intermediate seal portion of the pin fits into the intermediate seal portion of the box with an interference fit. In the fastened state, the outer male threaded portion of the pin is screwed into the outer female threaded portion of the box. Also preferably, the inner shoulder surface of the pin contacts the inner shoulder surface of the box in the fastened state.

The pin further includes a pin intermediate risk section located near the tapered large diameter end of the inner male threaded portion. The position of the intermediate dangerous section of the pin is the position where the cross-sectional area is the smallest in the vicinity of the large-diameter side end of the inner male thread portion. Typically, the cross section of the pin at the position where the contact between the load flank surfaces of the inner male threaded portion and the inner female threaded portion ceases in the fastened state is the intermediate dangerous cross section of the pin.

The box consists of a box dangerous cross section located near the end of the tapered shape of the inner female thread (that is, the inner side of the box) and a box intermediate dangerous cross section located near the end of the tapered shape of the outer female thread on the small diameter side. and cross section. The position of the box critical cross-section is the position where the cross-sectional area is the smallest in the vicinity of the small-diameter side end of the internal female thread. Typically, the cross section of the box at the position where the load flank surfaces of the inner male threaded portion and the inner female threaded portion are no longer in contact with each other in the fastened state is the box critical cross section. Further, the position of the box intermediate dangerous cross-section is the position where the cross-sectional area is the smallest in the vicinity of the small-diameter side end of the external female thread. Typically, the cross section of the box at the position where the contact between the load flank surfaces of the outer male threaded portion and the outer female threaded portion ceases to be the box middle dangerous cross section in the fastened state.

PICCSA is the area of the pin intermediate risk section when the pin and box are not fastened, BICCSA is the box intermediate risk cross section area when the pin and box are not fastened, and the area of the box intermediate risk cross section when the pin and box are not fastened. When the area of the box critical section is BCCSA, the pin and box have the following relationship:
PICCSA + BICCSA > BCCSA
0.70≦PICCSA/BICCSA≦0.95
is preferably satisfied.

In the present disclosure, each risk section does not include the thread section of each tapered thread, and the area of each risk section excludes the threads of the complete thread portion and the incomplete thread portion of each tapered thread. Defined as the cross-sectional area of the pin or box of the part.

Also, the inner peripheral surface of the pin may have substantially the same inner diameter throughout the nose, inner seal, inner external threads, intermediate shoulder, intermediate seal and outer external threads. The inner seal portion and the nose portion may have inner peripheral surfaces that gradually decrease in diameter toward the distal end side.

Also, the outer peripheral surface of the box may have substantially the same outer diameter across the inner seal portion, the inner female threaded portion, the intermediate shoulder, the intermediate seal portion and the outer female threaded portion.

The above PICCSA/BICCSA is more preferably 0.73 or more, still more preferably 0.75 or more.

The above PICCSA/BICCSA is more preferably 0.91 or less, still more preferably 0.90 or less.

In one embodiment, the axial distance between the pin mid-risk section of the pin and the mid-seal portion is greater than the axial distance between the box mid-danger cross-section and the mid-seal portion of the box. As a result, it is possible to reduce the amount of diametrical contraction deformation due to external pressure acting on the intermediate seal portion of the box, and an improvement in sealing performance against external pressure at the intermediate seal portion can be expected.

Preferably, the pin is provided at the end of the steel tube and the box is provided at the coupling. More specifically, the steel tube has a tube body and the pin extends axially from the end of the tube body. The pipe body is the portion that is not inserted into the box in the fastened state. In this case, it is preferable to make the box critical cross-sectional area BCCSA of the coupling larger than the cross-sectional area A of the pipe main body of the steel pipe. According to this, a tensile strength equal to or greater than the tensile strength of the steel pipe can be ensured in the threaded joint portion. Moreover, the present invention can be suitably implemented as a threaded joint for steel pipes having a wall thickness of 20 mm or more. Here, the wall thickness of the pipe main body being 20 mm or more means that the minimum wall thickness of the pipe main body is 20 mm or more.

[Configuration of threaded joint for steel pipe]
Referring to FIG. 1, a threaded joint 1 for steel pipes according to the present embodiment is a coupling-type threaded joint that includes a tubular pin 2 and a tubular pin 2 that is screwed into and fastened with the pin 2. box 3; In the illustrated example, the pin 2 is provided at the end of the oil well pipe T, and the box 3 is provided at the end of the coupling C. As shown in FIG.

The pin 2 extends axially from the end of the main body of the oil country tubular good T to the tip side. Pin 2 includes a nose portion 21 , an inner seal portion 22 , a tapered inner male threaded portion 23 , an intermediate shoulder portion 24 , an intermediate seal portion 25 and a tapered outer male threaded portion 26 . Each section 21-26 is tubular or ring-shaped, respectively.

The nose portion 21 is provided on the distal end side of the inner seal portion 22 and constitutes the distal end portion of the pin 2 . The outer peripheral surface of the nose portion 21 continues to the inner seal surface provided on the outer periphery of the inner seal portion 22 and extends toward the tip side. The outer peripheral surface of the nose portion 21 may be a tapered surface whose diameter gradually decreases toward the distal end side. Further, the outer peripheral surface of the nose portion 21 may have a shape in which the tapered surface described above and the outer peripheral surface of a rotating body obtained by rotating a curved line such as an arc around the tube axis CL are combined. A tip surface of the nose portion 21 constitutes an inner shoulder surface of the pin 2 . This shoulder surface is an annular surface that is substantially perpendicular to the tube axis CL, but the outer peripheral side thereof may be slightly inclined toward the tip side of the pin 2 .

The inner seal portion 22 is provided on the distal end side of the inner male thread portion 23 . The inner seal portion 22 is provided between the nose portion 21 and the inner male thread portion 23 . The inner seal surface is formed on the outer peripheral surface of the inner seal portion 22 so that the diameter of the inner seal surface decreases toward the distal end side. The vertical cross-sectional shape of this sealing surface may be linear, arc-shaped, or a combination of a straight line and an arc.

The inner male threaded portion 23 is provided between the inner seal portion 22 and the intermediate shoulder portion 24 . The outer peripheral surface of the inner male threaded portion 23 is provided with a tapered thread whose diameter gradually decreases toward the distal end side.

The intermediate shoulder portion 24 is provided between the inner male thread portion 23 and the intermediate seal portion 25, and is provided near the axial center of the pin 2 in the structure shown in FIG. The intermediate shoulder portion 24 has an intermediate shoulder surface substantially perpendicular to the tube axis CL. The intermediate shoulder surface is formed by a stepped portion provided on the outer periphery of the pin 2 between the inner male thread portion 23 and the intermediate seal portion 25 . The intermediate shoulder surface faces distally. Further, the intermediate shoulder surface may be slightly inclined toward the tip side of the pin 2 on the outer peripheral side or the inner peripheral side.

The intermediate seal portion 25 is provided between the intermediate shoulder portion 24 and the external male thread portion 26 . An intermediate seal surface is formed on the outer peripheral surface of the intermediate seal portion 25, the diameter of which decreases toward the tip side. The vertical cross-sectional shape of this sealing surface may be linear, arc-shaped, or a combination of a straight line and an arc.

The outer peripheral surface of the external male threaded portion 26 is provided with a tapered thread whose diameter gradually decreases toward the distal end side. The outer diameter and inner diameter of the base end portion of the external male threaded portion 26 are substantially equal to the outer diameter and inner diameter of the main body of the oil country tubular goods T. As shown in FIG.

The box 3 includes a concave portion 31 corresponding to the nose portion 21, an inner seal portion 32 corresponding to the inner seal portion 22, a tapered inner female thread portion 33 corresponding to the inner male thread portion 23, and an intermediate shoulder portion corresponding to the intermediate shoulder portion 24. 34 , an intermediate sealing portion 35 corresponding to the intermediate sealing portion 25 , and an external female threaded portion 36 corresponding to the external male threaded portion 26 . Each section 31-35 is tubular or ring-shaped, respectively.

The recessed portion 31 is provided on the inner side of the inner seal portion 22 . That is, the recessed portion 31 is provided at the innermost portion of the box 3 . The nose 21 of the pin 2 is inserted into the recess 31 in the fastened state. The inner side end surface of the concave portion 31 functions as an inner shoulder surface with which the inner shoulder surface of the pin 2 contacts in the fastened state. A gap may be formed between the inner shoulder surface of the pin 2 and the inner shoulder surface of the box 3 without contacting each other in the fastened state.

The inner seal portion 32 is provided on the back side of the inner male thread portion 33 . The inner seal portion 32 is provided between the recess 31 and the inner female thread portion 33 . An inner seal surface is formed on the inner peripheral surface of the inner seal portion 32 so that the diameter of the inner seal surface decreases toward the inner side of the box (in other words, the tip side of the pin). The vertical cross-sectional shape of this sealing surface may be linear, arc-shaped, or a combination of a straight line and an arc.

The inner female threaded portion 33 is provided between the inner seal portion 32 and the intermediate shoulder portion 34 . The inner peripheral surface of the inner female threaded portion 33 is provided with a tapered thread whose diameter gradually decreases toward the inner side of the box.

The intermediate shoulder portion 34 is provided between the inner female threaded portion 33 and the intermediate seal portion 35, and is provided near the axial center of the box 3 in the structure shown in FIG. The intermediate shoulder portion 34 is located on the tip side of the intermediate shoulder portion 24 of the pin 2 in the fastened state. The intermediate shoulder portion 34 has an intermediate shoulder surface substantially perpendicular to the tube axis CL. This intermediate shoulder surface is constituted by a stepped portion provided on the inner circumference of the box 3 between the inner female thread portion 33 and the intermediate seal portion 35 . The intermediate shoulder surface axially opposes the intermediate shoulder surface of the pin 2 . In the example shown, the axial length of the intermediate shoulder 24 of the pin 2 is greater than the axial length of the intermediate shoulder 34 of the box 3 . It should be noted that the axial length of the intermediate shoulder portion 34 of the box 3 may be greater than the axial length of the intermediate shoulder 24 of the pin 2 .

The intermediate seal portion 35 is provided between the intermediate shoulder portion 34 and the external female thread portion 36 . The inner peripheral surface of the intermediate seal portion 35 is formed with an intermediate seal surface whose diameter decreases toward the back of the box. The vertical cross-sectional shape of this sealing surface may be linear, arc-shaped, or a combination of a straight line and an arc.

The inner peripheral surface of the outer female threaded portion 36 is provided with a tapered thread whose diameter gradually decreases toward the inner side of the box.

The inner female threaded portion 33 can be screwed into the inner male threaded portion 23, and in a fastened state, the inner female threaded portion 33 is fitted and closely attached to each other, resulting in an interference fit. The external female threaded portion 36 can be screwed into the external male threaded portion 26, and in a fastened state, the external female threaded portion 36 is fitted and closely attached to each other, resulting in an interference fit state. The inner seal portions 22 and 32 come into contact with each other as the pin 2 is screwed in, and are tightly fitted together in the tightened state, resulting in an interference fit. The intermediate seal portions 25 and 35 also come into contact with each other as the pin 2 is screwed in, and are tightly fitted together in the tightened state, resulting in an interference fit. As a result, the inner seal surfaces of the inner seal portions 22 and 32 and the intermediate seal surfaces of the intermediate seal portions 25 and 35 are strongly pressed against each other to form metal seals. In the threaded joint having such a configuration, the inner seal portions 22 and 32 are tightly fitted to each other to ensure sealing performance mainly against internal pressure. In addition, due to the tight fit between the intermediate seal portions 25 and 35, sealing performance is mainly ensured against external pressure.

The shoulder surfaces at the tips of the pin 2 and the box 3 and the intermediate shoulder surfaces of the pin 2 and the box 3 come into contact with each other and are pressed together as the pin 2 is screwed in, and serve as stoppers that restrict the screwing of the pin 2. responsible for In addition, it is also possible to configure so that either the tip shoulder surface or the intermediate shoulder surface of the pin 2 does not come into contact with the box 3 .

A gap is formed between the outer peripheral surface of the nose portion 21 of the pin 2 and the inner peripheral surface of the recessed portion 31 of the box 3 in the fastened state. As a result, even if an excessive tensile load is applied and the inner seal surfaces of the pin 2 and the box 3 move relatively slightly in the axial direction in a direction in which the contact pressure between the inner seal surfaces of the pin 2 and the box 3 loosens, the inner seal surfaces of the pin 2 The contact pressure between the inner sealing surfaces of the pin 2 and the box 3 can be suppressed from being lowered by elastically restoring the sealing surfaces together with the nose portion 21 .

As shown in FIG. 1 , the pin 2 further includes a pin intermediate critical section (PICCS) located near the tapered large diameter side (ie, proximal side) end of the inner male threaded portion 23 . In addition, the box 3 has a box critical cross section (BCCS) located near the end on the smaller diameter side of the inner female threaded portion 33 (that is, the inner side of the box) and the smaller diameter side of the outer female threaded portion 36 (that is, on the smaller diameter side of the box). and a Box Intermediate Critical Section (BICCS) located near the end of the box (back side of the box). The pin 2 and the box 3 are configured such that the area of these dangerous cross sections satisfies the following formula.

〔formula〕
PICCSA + BICCSA > BCCSA
0.70≦PICCSA/BICCSA≦0.95
In the above formula, PICCSA is the area of PICCS, BICCSA is the area of BICCS, and BCCSA is the area of BCCS.

Further, in the threaded joint of this embodiment, the axial distance between the pin intermediate critical cross-section (PICCS) of the pin 2 and the intermediate seal portion 25 is the same as the box intermediate critical cross-section (BICCS) of the box 3 and the intermediate seal portion 35. greater than the axial distance between More preferably, the axial distance between the pin intermediate critical cross section (PICCS) and the intermediate seal portion 25 can be greater than the axial distance between the intermediate shoulder surface and the intermediate seal portion 25 . When the intermediate seal surfaces of the intermediate seal portions 25 and 35 are in surface contact with each other, the reference position for the axial distance may be the axial center position of the surface contact range.

Furthermore, the box critical cross-sectional area BCCSA of the coupling C is larger than the cross-sectional area A of the main body of the oil country tubular good T.

[Actions and effects of threaded joints for steel pipes]
According to the present embodiment, excellent sealing performance can be ensured in a threaded joint for steel pipes having a two-step thread structure including an intermediate seal and an intermediate shoulder. The reason for this will be explained in detail below.

Since the sum of the areas of the pin intermediate critical cross section (PICCS) and the box intermediate critical cross section (BICCS) PICCSA + BICCSA is larger than the area BCCSA of the box critical cross section (CCS), the tensile strength near the intermediate seal portions 25 and 35 of the threaded joint 1 is can be secured.

Also, although the pin intermediate critical cross-sectional area PICCSA is smaller than the box intermediate critical cross-sectional area BICCSA, by providing the intermediate seal portion 25 of the pin 2 between the intermediate shoulder portion 24 and the external male thread portion 26, the intermediate seal portion 25 can be A large cross-sectional area can be ensured, and deformation of the intermediate seal portion 25 with a reduced diameter can be suppressed when a large external pressure is applied.

Furthermore, by setting the area ratio PICCSA/BICCSA of the pin intermediate risk section (PICCS) to the box intermediate risk cross section (BICCS) to 0.70 or more and 0.95 or less, even when a tensile load is applied, the pin intermediate risk section The amount of extension of the pin 2 in the vicinity of the cross section and the amount of extension of the box 3 in the vicinity of the intermediate dangerous cross section of the box are set to be approximately the same, thereby preventing the intermediate seal surface of the pin 2 and the intermediate seal surface of the box 3 from being displaced in the axial direction. In addition, bending deformation in the vicinity of the sealing surface can be suppressed, and the amount of seal interference is substantially maintained, so that deterioration of sealing performance can be prevented.

The present disclosure can be applied not only to coupling-type threaded joints but also to integral-type threaded joints. Also, the tip of the pin may not be provided with a nose portion. Also, the tapered threads of each threaded portion are trapezoidal threads in the embodiment shown in FIG. 1, but may be API round threads, API buttress threads, or wedge threads. In addition, the present disclosure is not limited to the above embodiments, and various modifications are possible without departing from the scope of the present disclosure.

In order to confirm the effects of the threaded joint for steel pipes according to the present embodiment, a numerical analysis simulation was performed using the elasto-plastic finite element method.

<Test conditions>
For coupling-type threaded joints for oil well pipes, multiple specimens (analysis models) were created with different area ratios PICCSA/BICCSA between PICCS and BICCS, and elasto-plastic finite element analysis was performed on each specimen to determine the performance. compared the difference between Each specimen is a coupling-type threaded joint having the basic structure shown in FIG. Common test conditions are shown below.

(1) Analysis model The created analysis model is a two-dimensional axisymmetric model. That is, the screw threads of the screw portions 23, 26, 33, and 36 do not have a helical structure. A dimensional axisymmetric model was created. Various papers and the like have verified that the results of FEM analysis using such a two-dimensional axisymmetric model can be regarded as equivalent to the sealing performance of an actual product in the evaluation of sealing performance.

(2) Dimensions of steel pipe 7-5/8 [inch] × 1.06 [inch] (outer diameter 193.7 [mm], wall thickness 27.0 [mm])

(3) Steel pipe grade API standard P110 (low alloy steel with a nominal yield stress of 110 [ksi])
In the FEM analysis, each specimen is modeled so that the material is an isotropically hardened elastoplastic body, the elastic modulus is 210 GPa, and the yield strength is 110 ksi (758.3 MPa) at 0.2% proof stress. It was used.

(4) Screw dimensions assumed by the two-dimensional axisymmetric model Screw pitch: 5.08 mm
Flank angle of load surface: -3°
Flank angle of insertion surface: 10°
Insertion surface clearance: 0.15 mm

(5) Box critical cross-sectional area (BCCSA)
106% of the pipe body cross-sectional area A of the steel pipe

<Evaluation method>
First, an analysis simulating fastening was performed on each specimen, and then the sealing performance was evaluated when a repeated combined load simulating the ISO13679:2011 Series A test was applied to the model of the fastening state. The seal performance is the minimum value obtained by dividing the contact force per circumferential unit length between the inner seal surfaces of the pin and box by the applied internal pressure in the internal pressure cycle (first and second events) of the cyclic load history. In addition, in the external pressure cycle of repeated load history (third and fourth events), the value obtained by dividing the contact force per circumferential length between the intermediate seal surfaces of the pin and box by the applied external pressure was evaluated by the minimum value of It means that the larger these values are, the better the sealing performance of each sealing surface. In addition, the sealing performance was evaluated on the following two levels using relative values where the performance in internal and external pressure cycles of test sample #9 (the area ratio PICCSA/BICCSA of PICCS and BICCS is 1) was taken as 1.000.
○: internal pressure sealing performance is 1.000 or more and external pressure sealing performance is 1.100 or more ×: internal pressure sealing performance is less than 1.000, or external pressure sealing performance is less than 1.100

Table 1 summarizes the test conditions and evaluation of each specimen.

Specimens #1 to #8 are embodiments according to the present disclosure, and specimens #9 and #10 are comparative examples. Specimens #2 to #5 and #10 have the same box as that of specimen #1, but have different pin dimensions. Specimens #6 to #9 have the same pin as that of specimen #1, but the dimensions of the box are changed. Also, the meanings of the terms in the table are as follows.
PIN: Pin, BOX: Box, ICCSA: Intermediate dangerous cross-sectional area of pin and box, Ratio of ICCSA: Ratio of each intermediate dangerous cross-sectional area to cross-sectional area A of steel pipe main body, Ratio of PICCSA + BICCSA: Pin middle of each specimen Sum of critical cross-sectional area ratio and box intermediate critical cross-sectional area ratio, PICCSA/BICCSA: ratio of pin intermediate critical cross-sectional area to box intermediate critical cross-sectional area of each specimen, Load Cycle: type of load cycle, External: external pressure cycle, Internal : internal pressure cycle, Sealability: sealing performance, Eval. :Evaluation results

Note that the cross-sectional areas and area ratios shown in Table 1 are in a state in which the pin and the box are not fastened.

Based on the test results shown in Table 1, FIG. 2 shows the relationship between the ratio of the intermediate critical cross-sectional area of the pin to the intermediate critical cross-sectional area of the box and the sealing performance. Also, FIG. 3 shows the relationship between the sum of the intermediate critical cross-sectional area ratios of the pin and the box and the sealing performance.

As is clear from FIG. 2, the sealing performance against external pressure is remarkably improved to 1.10 or more when PICCSA/BICCSA is in the range of 0.70 to 0.95, and particularly in the range of 0.73 to 0.91. The external pressure sealing performance is maintained at 1.13 or higher. Test pieces #1 to #8 have such high external pressure sealing performance, and the sum of PICCSA and BICCSA is the box critical cross-sectional area (as described above, 106% of the cross-sectional area A of the steel pipe main body). ) and the PICCSA/BICCSA is 0.7 or more and 0.95 or less, so the amount of deformation in the vicinity of the intermediate seal is small, and good sealing performance is obtained.

In specimen #9, although PICCSA+BICCSA was larger than the box critical cross-sectional area, PICCSA/BICCSA was larger than 0.95, so the amount of deformation in the vicinity of the intermediate seal was large, presumably resulting in poor sealing performance.

In specimen #10, PICCSA+BICCSA was smaller than the critical cross-sectional area of the box and PICCSA/BICCSA was smaller than 0.7.

From the above results, according to the present disclosure, it was demonstrated that a threaded joint for steel pipes having a two-stage screw structure including an intermediate seal can obtain good sealing properties against internal pressure and external pressure.

2: pin, 21: nose portion, 22: first seal portion, 23: first male screw portion,
24: intermediate shoulder portion, 25: second seal portion, 26: second male thread portion,
3: Box, 31: Recessed portion, 32: Third seal portion, 33: First female screw portion,
34: intermediate shoulder portion, 35: fourth seal portion, 36: second female thread portion,
PICCS: Pin Intermediate Risk Section, PICCSA: Pin Intermediate Risk Section BICCS: Box Intermediate Risk Section, BICCSA: Box Intermediate Risk Section BCCS: Box Risk Section, BCCSA: Box Critical Section A: Cross-sectional Area of Pipe Body of Steel Pipe

Claims (9)

A threaded joint for steel pipes, comprising a tubular pin and a tubular box, wherein the pin is screwed into the box to fasten the pin and the box,
The pin includes a first male threaded portion, an intermediate seal portion, and a second male threaded portion in order from the tip end side of the pin, and a tip end portion is provided between the first male thread portion and the intermediate seal portion of the pin. A facing intermediate shoulder surface is provided,
The box includes, in order from the innermost side of the box, a first female threaded portion into which the first male threaded portion of the pin is fitted in a fastened state, an intermediate seal portion into which an intermediate seal portion of the pin is fitted in a fastened state, and a second female threaded portion into which the second male threaded portion of the pin is fitted in a fastened state; an intermediate shoulder surface is provided between the first female threaded portion and an intermediate seal portion of the box; a surface axially opposed to an intermediate shoulder surface of said pin;
the pin further includes a pin intermediate risk section positioned near the proximal end of the first male threaded portion;
The box further includes a box dangerous cross-section located near the far end of the first female thread and a box intermediate dangerous cross-section located near the far end of the second female thread,
PICCSA is the area of the pin intermediate risk section when the pin and the box are not fastened, BICCSA is the area of the box intermediate risk cross section when the pin and the box are not fastened, and the pin and the box When BCCSA is the area of the box critical cross-section when the is not fastened, the pin and the box have the following relationship:
PICCSA + BICCSA > BCCSA
0.70≦PICCSA/BICCSA≦0.95
A threaded joint for steel pipes that satisfies 2. The threaded joint for steel pipes according to claim 1, wherein PICCSA/BICCSA is 0.73 or more. 3. The threaded joint for steel pipes according to claim 1 or 2, wherein PICCSA/BICCSA is 0.91 or less. 4. The threaded joint for steel pipes according to claim 1, wherein the pin further comprises an inner seal portion provided on the tip side of the first male threaded portion, and the box comprises the first male threaded portion. A threaded joint for steel pipes, further comprising an inner seal portion provided on the inner side of the joint, wherein the inner seal portion of the pin fits into the inner seal portion of the box in a fastened state. 5. The threaded joint for steel pipes according to claim 4, wherein the pin further includes a nose portion provided on the tip side of the inner seal portion of the pin, and the box is on the inner side of the inner seal portion of the box. A threaded joint for steel pipes, further comprising a recess provided in a threaded joint for steel pipes, wherein the nose portion is inserted into the recess in a fastened state. The threaded joint for steel pipes according to any one of claims 1 to 5, wherein the pin further has an inner shoulder surface formed by a tip surface of the pin, and the box is axially attached to the inner shoulder surface of the pin. A threaded joint for steel pipes, further comprising directionally opposed inner shoulder surfaces. The threaded joint for steel pipes according to any one of claims 1 to 6, wherein the pin extends axially from the end of the pipe body of the steel pipe to the tip side, and the box is provided in the coupling. , threaded joints for steel pipes. 8. The threaded joint for steel pipes according to claim 7, wherein the box critical cross-sectional area BCCSA of the coupling is larger than the cross-sectional area A of the pipe body of the steel pipe. 9. The threaded joint for steel pipes according to claim 7 or 8, wherein the pipe body of the steel pipe has a wall thickness of 20 mm or more.

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